High-power fiber lasers require fibers having large core diameters and large mode-field diameters. Numerous methods exist to achieve good beam quality from large-core fibers; however, many of these so-called large-mode-area (LMA) fibers are difficult to make and are expensive.
A prime example of structures that achieve good beam quality from large-core fibers are photonic crystal fibers (PCFs). Such PCFs deliver excellent beam quality (which have been demonstrated only with optical signals at wavelengths of about one micron) in PCFs with large mode-field diameters (also called large mode-field area of large-mode-area (LMA) fibers); however, PCFs are extremely difficult to produce, difficult to cleave, and up to now impossible to fusion splice. Therefore, such fibers do not enable a true all-fiber assembly, which is very important when building highly robust laser systems.
In contrast, standard step-index fibers are simple to cleave and fusion splice and allow building of all-fiber laser systems. However, such fibers have typically degraded beam quality due to their high numerical aperture (NA), which is typically in the range of 0.15-0.22 (wherein NA is equal to the sine of the acceptance angle Θa).
A gain-producing fiber (also called a “gain fiber” herein) is an optical fiber that includes one or more light-amplifying species, typically rare-earth dopants, which absorb and store energy from pump light (optical radiation), and then that absorbed stored energy amplifies an optical signal beam.
U.S. Patent Application Publication 2008/0180787 (which issued as U.S. Pat. No. 7,916,386 on Mar. 29, 2011) by DiGiovanni et al. was filed Jul. 31, 2008 titled “High power optical apparatus employing large-mode-area, multimode, gain-producing optical fibers,” and is incorporated herein by reference. DiGiovanni et al. describe optical apparatus that includes a multimode, gain-producing fiber for providing gain to signal light propagating in the core of the fiber, and a pump source for providing pump light that is absorbed in the core, characterized in that (i) the pump source illustratively comprises a low brightness array of laser diodes and a converter for increasing the brightness of the pump light, (ii) the pump light is coupled directly into the core, and (iii) the area of the core exceeds approximately 350 μm2. In one embodiment, the signal light propagates in a single mode, and the pump light co-propagates in at least the same, single mode, both in a standard input fiber before entering the gain-producing fiber, and a mode expander is disposed between the input fiber and the gain fiber. In another embodiment, multiple pumps are coupled into the core of the gain fiber. The pumps may generate light of the same wavelength or of different wavelengths. In accordance with a particular embodiment of the invention, amplification of nanosecond optical pulses was demonstrated at 1545 nm in a single-clad Er-doped fiber having a core area of 875 μm2, wherein the core was pumped by a high-brightness Raman laser at 1480 nm; and the pulses had a record peak power of several hundred kW. DiGiovanni et al. do not say anything about polarization.
There is a need for improved laser and amplifier systems, particularly large-mode-area (LMA) optical-fiber lasers and/or optical-fiber amplifiers, and optionally having delivery fibers, wherein the systems have improved beam quality (e.g., such that M2 approaches 1, and/or such that output beam is polarized) and have very high average power.